resolved: Implement continuous mDNS querying as per RFC6762 5.2 (#22532)

Allow for mDNS service/domain/types browsing.
A client can connect to the backend via varlink and receive updates as
the requested service becomes available.

The interval between the first two queries MUST be at least one second,
the intervals between successive queries MUST increase by at least a
factor of two.
When the interval between queries reaches or exceeds 60 minutes, a
querier MAY cap the interval to a maximum of 60 minutes, and perform
subsequent queries at a steady-state rate of one query per hour.

Cache maintenance performed by issuing queries at 80, 85, 90, 95% of the
TTL, and updating/removing the records depending on replies.

TODO:
Improve the DNS transaction logic when multiple clients subscribe to the
same service, ensuring that continuous queries are optimized.

<!-- devel-freezer =
{"comment-id":"1625154850","freezing-tag":"v256-rc2"} -->